A radiation detector is used, for example, in a PET (Positron Emission Tomography) scanner. The radiation detector used in the PET scanner detects a pair of gamma rays generated in association with electron/positron pair annihilation within a specimen into which a positron emission isotope (RI radiation source) is injected to make a flight in the reverse direction of each other. The PET scanner detects a pair of gamma rays by a coincidence method which utilizes a plurality of radiation detectors, thereby accumulating the coincidence counting information thereof to prepare a histogram. Then, on the basis of the histogram, the PET scanner reconstructs an image indicating a spatial distribution of generation frequencies of a pair of gamma rays at a space to be measured. The PET scanner has played an important role in nuclear medicine, etc., and can be used for conducting research on functions of the body and high-level functions of the brain.
As a radiation detector favorably used in the above-described PET scanner and others, there is available that which has a scintillator and a photodetector. The scintillator absorbs incident gamma rays and generates scintillation light. The photodetector is attached to a surface of the scintillator to detect scintillation light. The above constitution is able to identify a gamma-ray incident position in the scintillator and the amount of gamma rays.
Patent Document 1 has disclosed a radiation detector having a scintillator and a photodetector. The scintillator described in the literature has internally a light guide region which restricts a scintillation-light advancing direction. Examples of the above-described light guide region include a boundary face between media substantially different in refractive index, reflection films, bubbles, defects, and crystal defects such as crystal grain boundary.
Further, Patent Document 2 has disclosed a technology in which multi-photon absorption by femtosecond pulse laser light is used to form an amorphous-structured modified region different in refractive index from the surrounding region thereof inside an object to be processed such as a silicon base plate, silica glass or sapphire.    Patent Document 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-532864    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-293735